This invention relates generally to the manufacture of biosensor electrodes.
An enzyme electrode is a biomedical sensor that can be used to determine the concentration of certain biochemicals rapidly and with considerable accuracy. Enzyme electrodes are used to detect glucose, urea, uric acid, various alcohols, and a number of amino acids when used in certain well-defined situations.
Glucose sensors suitable for in vivo use can be prepared by depositing a glucose sensitive enzyme, such as glucose oxidase, onto an electrode via an electromotive plating process. The substrate is immersed in a bath comprising glucose oxidase, a stabilizing protein, a surfactant and a buffer for conductivity and stability of the protein solution. The enzyme is deposited onto the electrode potentiometrically.
Such methods require considerable volumes of solution for immersion of the electrodes and can result in protein contamination of counter or reference electrodes if present in the array, which, in turn, leads to signal interference. In addition, the concentrations of solution components can vary over a series of electrode immersions and plating cycles from the same bath. There is thus a need for improved methods of plating enzyme onto electrodes that can provide greater efficiency and reproducibility in the plating process, as well as providing adaptability to automated plating processes.
To overcome the limitations in the prior art described above, and to overcome other limitations that will become apparent upon reading and understanding the present specification, the invention provides a method for depositing an electroactive species onto an electrode. In one embodiment, the method comprises partially dispensing a solution containing the electroactive species from a microdispenser so as to form a hanging drop of the solution. The method further comprises contacting the electrode with the hanging drop of the solution, wherein the electrode is electrically coupled with the microdispenser so as to form an electrochemical cell, and applying a potential to the electrochemical cell. The application of the potential results in deposition of the electroactive species onto the electrode.
In yet another embodiment, a microdispensing device, such as a BioJet(copyright), may be used to deposit a fine bead of conductive plating solution on the first or xe2x80x9cworking electrodexe2x80x9d thus avoiding contact with the reference or counter electrodes in the sensor configuration. This method further comprises placing one or more electrodes into the bead and applying a potential to the electrochemical cell. This type of microdispensing device can be readily adapted for automation of the method for plating a plurality of working electrodes, providing efficiency of both time and costs, as well as a high degree of accuracy.
The method of the invention eliminates the need for immersion of the electrode in a bath, reduces the volume of solution required by a factor of at least 10-100, and avoids uneven depletion of various components of the solution over successive applications. The method reduces costs, provides for increased reproducibility in the plating process and avoids contamination of the solution. Because of the direct placement of solution onto the electrode and greater control over the deposition as a consequence of current density, a smaller amount of stabilizing protein is required, resulting in greater activity and reduced plating times.
In one embodiment, the invention provides an apparatus for depositing an electroactive species onto an electrode. The apparatus comprises a microdispenser capable of dispensing a solution containing the electroactive species so as to form a hanging drop of the solution, an electrode holder capable of placing the electrode in electrical contact with the microdispenser so as to form an electrochemical cell, and a potentiometer disposed between the microdispenser and the electrode holder. Examples of electroactive species include, but are not limited to, enzymes, such as glucose oxidase, lactate oxidase, and amino acid oxidase, as well as metallic solutions, such as a platinum salt (e.g., hydrogen hexachloroplatinate) solution.
In one embodiment, the apparatus is used for depositing an electrode surface, such as platinum black onto a conductive metal substrate such as gold, nickel, platinum, iridium or other suitable metal material. The solution to be microdispensed contains an appropriate metal salt or organometallic compound and forms a bead or drop to be deposited onto an electrode. The electrode holder is used to place an inert electrode in electrical contact with the solution drop or bead, and a potentiometer is disposed between the microdispenser and the electrode so as to form an electrochemical cell.